For a transmitter whose output can be controlled variably, the power efficiency and the linearity in a transmission function are evaluated as indicators for measuring the performance of the apparatus. The power efficiency and the linearity in the transmission function are the most important indicators to represent the performance of the apparatus particularly in a high-frequency modulation transmission machine such as a mobile telephone.
An amplifier of class A operation is widely used as an amplifier at the last stage of such a high-frequency modulation transmission machine. The class A amplifier has small distortion, namely, is excellent in linearity, but provides small power efficiency because it consumes power accompanying a DC bias component at all times.
Then, designed as a method of operating a power amplifier with a high degree of efficiency is a method of changing drain voltage or collector voltage (power supply voltage) in response to the amplitude component of a baseband signal for amplification using the saturation region of the input/output power characteristic of a transistor. For example, when average output power is changed, the above-mentioned power supply voltage is changed in proportion to any desired average output power. As this kind of apparatus, for example, an output variable transmitter disclosed in Japanese Patent No. 3044057 (patent document 1) is available.
FIG. 11 is a block diagram to show the configuration of an output variable transmitter in a related art example. This output variable transmitter includes modulation input terminals 101 and 102, a carrier oscillator 104, a quadrature modulator 103 for performing quadrature modulation of outputs of the modulation input terminals 101 and 102 at the output frequency of the carrier oscillator 104, a high-frequency power amplifier 105, a transmission output terminal 106, an envelop generation circuit 107 for generating an envelope from the outputs of the modulation input terminals 101 and 102, a specification signal input terminal 112, a multilevel DC signal generation circuit 108 for inputting a signal for setting the average output level of the power amplifier 105 from the specification signal input terminal 112 and generating a DC signal corresponding to the input value of the signal, a multiplication circuit 109 for multiplying the output envelop of the envelop generation circuit 107 by output of the multilevel DC signal generation circuit 108, a voltage control circuit 110 for controlling drain voltage of the power amplifier 105 in response to output of the multiplication circuit 109, and a power supply terminal 111.
The quadrature modulator 103 modules the carrier supplied from the carrier oscillator 104 according to an I signal input from the modulation input terminals 101 and 102 and a Q signal orthogonal to the I signal. The envelop generation circuit 107 calculates an amplitude signal R of the I and Q signals. An output level specification signal corresponding to the transmission output level to output to the transmission output terminal 106 is input to the specification signal input terminal 112. The multilevel DC signal generation circuit 108 generates a DC signal in accordance with the output level specification signal from the specification signal input terminal 112.
The multiplication circuit 109 multiplies the output of the envelop generation circuit 107 by the output of the multilevel DC signal generation circuit 108. Accordingly, a signal proportional to the envelop of the modulated wave is obtained in the output of the multiplication circuit 109 and moreover the average value changes with transmission output. The voltage control circuit 110 changes drain voltage Vo of the power amplifier 105 in response to the output of the multiplication circuit 109. Consequently, the drain voltage of the power amplifier 105 is proportional to the envelop of the modulated wave and the average value changes with transmission output. Therefore, using the configuration of the polar coordinate modulation system as described above, the power amplifier 105 can perform linear amplification while holding the saturation state of high efficiency, and moreover transmission output can be made variable.
However, in the output variable transmitter in the related art example shown in FIG. 11, both amplitude modulation and transmission output control are always performed collectively as drain voltage control and thus the output control variable width of the transmission power is limited by the characteristic of the power amplifier 105. To install the output variable transmitter in the related art example in an actual mobile telephone, finite control voltage range (for example, 0.3 V to 3.0 V) and power amplifier gain variable width (for example, 20 dB/dec) can only be secured. Thus, the necessary transmission output level range cannot sufficiently be secured in a communication apparatus requiring a wide output control variable width as in a recent mobile telephone standard (for example, about 43 dB+α in EGPRS of European mobile telephone standard).
The frequency at which a mobile telephone operates with the maximum transmission output is comparatively low. This is caused by the fact that the transmission power is set low by a command of a base station to avoid interference and enhance the cell use efficiency. Therefore, to prolong the conversion-possible time of a mobile telephone, suppressing the power consumption not only at the maximum transmission output time, but also at the low transmission output time is an important problem.
Patent document 1: Japanese Patent No. 3044057 (p1-p20, FIG. 1)
Patent document 2: JP-A-2003-18026
Patent document 3: JP-A-2003-51751
Patent document 4: JP-A-2004-104194
Patent document 5: JP-A-2004-173249
Patent document 6: JP-A-3-276923